1. Field of the Invention
The present invention relates to an electrostatic image developer for developing an electrostatic image in an electrophotographic copier, a laser beam printer, an electrostatic recording apparatus or the like and to a process for forming an image using this electrostatic developer.
2. Description of the Prior Art
Heretofore, it has widely been conducted to obtain a copied image or a recorded image (hereinafter, both are inclusively referred to merely as “copied images”) using an electrophotographic copier or a laser beam printer utilizing electrophotography or an electrostatic recording apparatus utilizing electrostatic recording technology.
In the electrophotographic copiers or laser beam printers, formation of copied images utilizing electrophotography is commonly conducted as follows. That is, first, an electrostatic latent image carrier composed of a photoreceptor drum using an inorganic photoconductive material such as amorphous silicon or selenium or an organic photoconductive material (OPC) is positively or negatively charged by means of a charging device. Subsequently, this charged electrostatic latent image carrier is subjected to slit exposure or beam exposure to form thereby an electrostatic latent image on the electrostatic latent image carrier. The thus-formed electrostatic image is developed by a developer containing toner particles, and the developed toner image is transferred to a transfer material such as transfer paper. The transferred toner image is fixed by means of heat rolls, pressure rolls or the like to form a copied image.
As a process for developing the electrostatic image, there have been known (i) a dry-process two-component developing process such as a magnetic brush process or a cascade process which uses a two-component developer containing carrier particles of iron powder, glass powder or resin powder containing a magnetic material and toner particles containing a resin and a colorant as major components; (ii) a one-component developing process of conducting development using only toner particles and not using carrier particles; and (iii) a liquid developing process using an insulating carrier liquid. As the one-component developing process, a one-component magnetic developing process using as a developer an insulating magnetic toner having magnetic powder in the toner particles as described in U.S. Pat. No. 4,336,318 is popular. To the two-component developer and the one-component developer are further added, as needed, additives called external additives such as a fluidizing agent and an abrasive.
On the other hand, the toner partly remains on the electrostatic latent image carrier after transfer of the toner without being transferred. This residual developer is removed from the electrostatic latent image carrier by a cleaning member such as blade cleaning, fur brush cleaning or magnetic brush cleaning, and the thus-cleaned electrostatic latent image carrier is reused. In this occasion, the cleaning member is press-contacted against the electrostatic latent image carrier at a pressure necessary to remove the developer and hence, during repeated use of the electrostatic latent image carrier, the electrostatic latent image carrier is injured or toner particles or toner particle components fixedly deposit onto the electrostatic latent image carrier, leading to generation of deteriorated images. In order to avoid the phenomenon of the fixed deposition of the toner on the electrostatic latent image carrier or to remove the fixedly deposited substances, it has heretofore been widely conducted to incorporate an abrasive in the developer. As to the abrasive, various proposals have so far been made with respect to a using material, a combination of abrasives or a combination of the abrasive and other external additives.
For example, British Patent No. 1,402,010 describes to add a friction-reducing substance and an abrasive in the developer in order to prevent the phenomenon of fixed deposition of toner onto an electrostatic latent image carrier and to prevent generation of flaws on the electrostatic latent image carrier. This technique of using a developer containing both the friction-reducing substance and the abrasive is an effective technique for avoiding the fixed deposition phenomenon of the toner onto the electrostatic latent image carrier. However, addition of the friction-reducing substance in an amount enough to avoid the toner deposition phenomenon involves the defect that it would make it difficult to remove substances having a low electric resistance such as paper dust or an ozone adduct to be generated on, or adhered to, the surface of the electrostatic latent image carrier by repeating use of the photoreceptor. Particularly under an environment of a high temperature and a high humidity, the latent image on the photoreceptor would be injured by the substances having a low electric resistance. Also, selection of the amounts of the friction-reducing substance and the abrasive is delicate, and addition of the abrasive substance in an enough amount so as to stably remove deposits on the photoreceptor would injure the photoreceptor and injure a cleaning blade, resulting in cleaning failure.
As the abrasive material, there have been known colloidal silica, surface-modified oleophilic silica, aluminum silicate, surface-treated aluminum silicate, titanium dioxide, alumina, calcium carbonate, antimony trioxide, barium titanate, calcium titanate, strontium titanate, calcium silicate, magnesium oxide, zinc oxide, zirconium oxide, etc. by, for example, British Patent No. 1,402,010 and U.S. Pat. No. 3,900,588. Also, JP-A-55-57874 describes cerium oxide, aluminum oxide, silicon oxide, zinc oxide, chromium oxide, aluminum sulfate, calcium sulfate, barium sulfate, magnesium sulfate, etc. as abrasives. Further, JP-A-60-136752 proposes a process of forming an image by using a developer containing an inorganic fine powder of 0.2 to 30 m2/g in BET specific surface area measured according to the hydrogen absorption method generated by a sintering process, and U.S. Pat. No. 4,654,288 proposes an image-forming process by using a developer containing oxide series ceramic fine powder or non-oxide series ceramic fine powder such as silicon carbide, tungsten carbide, aluminum nitride or boron nitride of 0.1 μm or less in particle size, or a combination of the non-oxide series ceramic fine powder and a lubricant.
These conventionally known abrasives or the conventionally proposed techniques involve the problem that, in the case of using a photoreceptor such as amorphous silicon photoreceptor as the electrostatic latent image carrier, a sufficient cleaning effect can not be obtained. Also, in this occasion, when it is intended to avoid the toner deposition phenomenon onto the photoreceptor and obtain a sufficient cleaning effect, it is necessary to incorporate a large amount of inorganic fine powder in a developer. Also, in the case of using a silicon carbide fine powder or the like as an abrasive, there is observed filming of toner upon blade cleaning when used alone or, in case where the photoreceptor is an OPC or selenium series photoreceptor, there arises a problem that these photoreceptors are injured during repeated use thereof.
Further, in order to improve surface deterioration of amorphous silicon due to corona charge, JP-A-61-15154 proposes to conduct development while abrading the surface of the photoreceptor using a toner on the surface of which is externally added 0.05 to 5% by weight, per toner, of silicon carbide of 0.1 to 1 μm. However, in the case of using this developer on the surface of which is externally added silicon carbide, there is involved the problem, as described above, that fusing of the toner onto a photoreceptor drum is liable to take place upon blade cleaning and, in the case of reversal development, generation of white spots are observed in copied images due to the fusing of toner.